Magnetic Resonance Imaging (MRI) is a relatively new technique used in medicine to investigate in great detail the condition of the human body. The patient is typically placed supine on a moveable horizontal table which is moved through rapidly changing and intense magnetic fields. Detailed information on the interior of the body can be obtained by computer analysis of the magnetic resonance produced by the rapidly changing magnetic fields. In many instances, the information obtained from MRI is more detailed than the information available from the use of X-Ray and at less risk to the patient. Because the patient undergoing MRI is sick for one reason or another and because the MRI experience is often traumatic, it is important that the physiological state of the patient be monitored.
The room in which the MRI is performed is highly shielded, with monitoring equipment placed in an adjacent room. The patient connections are typically fed through the shielded wall to the monitoring equipment. While the shielding isolates the monitoring equipment from the intense, rapidly changing magnetic fields generated by the MRI equipment, the leadwires which extend from the patient and through the shielded wall are subjected to this intense magnetic field. The rapidly changing magnetic fields induce high level noise signals into the physiological monitoring leads, and these noise signals can interfere with MRI image, as well as the physiological monitoring image, and this can cause localized heating and skin burns under the physiological monitoring electrodes.
U.S. Pat. No. 4,991,580 discloses a method to improve the quality of an Electrocardiogram (ECG) by reducing the high level noise signals induced into the cardiac monitoring leads. This is accomplished by employing special circuitry in the room with the ECG monitoring equipment in order to reduce the noise and amplify the ECG signal. However, this patent does not address the problem of skin heating or burns that also result from the eddy currents that produce this noise.
U.S. Pat. No. 5,445,162 discloses a method to reduce the amount of magnetic metal and associated Electroencephalogram (EEG) equipment outside of the bore of the MRI magnet and possibly outside of the MRI room. The intent is to reduce distortion of the MRI and also to obtain an EEG during Magnetic Resonance Imaging. The patent mentions the induction of significant current flow in electrodes and wires within the magnetic field and the possibility of these currents producing localized heating or burns under EEG electrodes connected to the patient's scalp. However, this patent does not attempt to resolve the possibility of burns under the monitoring electrodes. A similar phenomena occurs under ECG electrodes connected to the patient and located within the rapidly changing magnetic fields of the MRI system.
Van Genderingen et al (Radiology 1989) discloses a system of using carbon fiber electrodes and leads for obtaining an ECG during cardiac gating with MRI. The primary goal was to reduce the amount of distortion of the gradient magnetic field and the corresponding image distortion and artifact caused when metallic electrodes and leads are used. The electrical resistance of the carbon fiber leads was around 1,000 ohms. While this resistance reduces distortion, it is insufficient to prevent patient burns.
U.S. Pat. No. 4,951,672 discloses a leadwire designed specifically to monitor ECG signals during MRI and provide protection from unwanted heating under monitoring electrodes. The patent recognizes the need for the ECG leadwires to have a high resistance in order to reduce the possibility of heating under the electrodes, but the solution was to mold metal film resistors, of the 33k ohm to 10k ohm range, in the electrode connector of the wire. Since this produced a hot spot at the electrode connector, the patent discloses the inclusion of resistor modules along the length of the wire. However, the resistor modules result in producing many hot spots. The patent also mentions that the resistance could be distributed over the length of the leadwire. From the drawings, this must mean that if enough resistor modules are added along the length of the wire, the heat can be evenly spread, thereby reducing the possibility of burns. In actual practice, this proposed solution cannot work well. As the multiple of resistor modules increases, so does the number of hot spots, and the resistance is not uniformly distributed along the length of the wire.
Pat. No. 4,280,507 discloses a "Patient Cable with Distributed Resistance Protection in Conductors". In this patent, the concept of distributed resistance is employed. However, the protection this device provides is protection of the ECG monitor circuitry from the extreme electrical pulses that are present at the patient's ECG electrode sites when the patient is defibrillated while still connected to the ECG monitor via the ECG electrodes and cable/wires. The device is not designed to provide patient protection from heating or burns. Moreover, the use of carbon loaded polymers for a distributed resistance conductor, limits how high of resistance can be employed and how tight of a resistance tolerance that can be maintained.